Clinton and Obama frequently address issues that hit home for unions -- wages, protection of the right to form unions, health insurance, cutting taxes for the middle class and reworking trade agreements that some union members blame for job loss.

A union's endorsement can give a candidate's campaign a significant boost because union members often act as ground troops to canvass neighborhoods and work the phones.

Speaking in Portage, Indiana, Clinton on Wednesday told workers "the American labor movement built the middle class."

"I will fight with you and for you. And no state needs a president more who understands the importance of manufacturing and the significance of the labor union movement than Indiana," she said in her speech at Duneland Falls Steel Workers Local Union Hall.

Obama last month told union members in Pennsylvania he's "ready to play offense for organized labor."

"It's time we had a president who didn't choke saying the word 'union.' It's time we had a Democratic nominee who doesn't just talk about unions in the primary," he said.

Don't Miss

Obama has the support of the Service Employees International Union, Teamsters and Unite Here, which represents hotel and restaurant workers.

Unite Here's leader, Bruce Raynor, said he does not trust Clinton when she says she will rework the North American Free Trade Agreement.

"We have a big problem with believing that the Clintons are committed to free trade policies that would protect American jobs. That worries us," he said. "Sen. Obama, on the other hand, has been with us from day one."

Clinton has faced skepticism because her husband, former President Bill Clinton, supports a free trade agreement with Colombia. Clinton insists she staunchly opposes it.

"I don't think any married couple I know agrees on everything, and we disagree on this," she said last month.